The present invention relates to automatic focusing systems for use in optical instruments. More particularly, the present invention relates to an improved automatic focusing system for use in an optical instrument which has an electrically powered, adjustable objective lens.
Automatic focusing systems to which the present invention is directed generally employ two optical systems operating on the principle of spatial image correlation. A fixed, reference optical system is electrically interconnected with a scanning optical system and appropriate lens driving mechanism to selectively adjust the objective lens.
More specifically, an image of a remote subject is directed to one side of a sensor, such as that described in U.S. Pat. No. 4,002,899, by the fixed, reference optical system. Another image of the subject is directed to the other side of the sensor by the scanning optical system which sweeps through a predetermined angle. When the two images are received by the sensor, a continuous control signal is produced representative of subject distance. When the sensor determines that the images from both optical systems are coincident, a peak pulse is generated and the lens is stopped. However, under low light conditions, problems arise in automatically focusing the lens.
In an improved automatic focusing system utilizing the spatial image correlation principle described above, it has been proposed to mechanically interconnect a scanning optical system with an objective lens to produce a change in the field of orientation of the scanning optical system as a function of the change in the focus position of the lens. The orientation of the objective lens is further controlled by a signal generator mounted on the scanner drive to produce a second reference signal. A third reference signal is generated by employing sliding contacts on the scanner drive to generate a signal representative of the orientation of the scanning optical system. Appropriate circuitry is employed to compare the reference signals and to selectively energize a focus motor to adjust the lens position such that the angle of orientation of the scanning optical system at the point of spatial image correlation bisects the full scanning angle.
In the latter described device, object distance may be determined under conditions of low light and high speed scanning, thus providing more accurate focusing over a wide variety of conditions. However, the latter device utilizes a number of precision moving parts which, for some applications, may be undesirable from a cost and/or reliability standpoint. In addition, wear of the precision parts may introduce focusing problems. Hence, prior proposals for automatically focusing an optical system are not entirely satisfactory for a broad range of applications.